Cathodic electrocoating compositions containing an anticrater agent

ABSTRACT

An improved aqueous electrocoating composition that contains an anticrater agent which is a reaction product of polyoxyalkylene diamine and isocyanto alkyl alkoxy silane which is hydrolyzed and has a number average molecular weight of about 2,000-5,000; electrodeposited finishes are formed that have a significant reduction in craters and are smooth and even finishes.

This is a division of application Ser. No. 08/772,613 filed Dec. 23,1996, U.S. Pat. No. 5,750,596.

BACKGROUND OF THE INVENTION

This invention is directed to a cathodic electrocoating composition andin particular to a cathodic electrocoating composition containing ananticrater agent which significantly reduces craters and improves thesmoothness of an electrodeposited film of the composition.

The coating of electrically conductive substrates by anelectrodeposition process, also called an electrocoating process is awell known and important industrial process. Electrodeposition ofprimers to automotive substrates is widely used in the automotiveindustry. In this process, a conductive article, such as an autobody oran auto part, is immersed in a bath of a coating composition of anaqueous emulsion of film forming polymer and acts as an electrode in theelectrodeposition process. An electric current is passed between thearticle and a counter-electrode in electrical contact with the aqueousemulsion, until a desired coating is deposited on the article. In acathodic electrocoating process, the article to be coated is the cathodeand the counter-electrode is the anode.

Resin compositions used in the bath of a typical cathodicelectrodeposition process also are well known in the art. These resinstypically are made from polyepoxide resins which have been chainextended and then an adduct is formed to include amine groups in theresin. Amine groups typically are introduced through reaction of theresin with an amine compound. These resins are blended with acrosslinking agent and then neutralized with an acid to form a wateremulsion which is usually referred to as a principal emulsion.

The principal emulsion is combined with a pigment paste, coalescentsolvents, water, and other additives to form the electrocoating bath.The electrocoating bath is placed in an insulated tank containing theanode. The article to be coated is the cathode and is passed through thetank containing the electrodeposition bath. The thickness of the coatingthat is deposited on the article being electrocoated is a function ofthe bath characteristics, the electrical operating characteristics ofthe tank, the immersion time, and the like.

The resulting coated article is removed from the bath after a set periodof time and is rinsed with deionized water. The coating on the articleis cured typically in an oven at sufficient temperature to produce acrosslinked finish on the article.

Cathodic electrocoating compositions, resin compositions, coating bathsand cathodic electrodeposition processes are disclosed in Jarabek et alU.S. Pat. No. 3,922,253 issued Nov. 25, 1975; Wismer et al U.S. Pat. No.4,419,467 issued Dec. 6, 1983; Belanger U.S. Pat. No. 4,137,140 issuedJan. 30, 1979 and Wismer et al U.S. Pat. No. 4,468,307 issued Aug. 25,1984.

A continuing problem with cathodic electrocoating compositions has beenthe presence of craters in the cured finish. An additive or agent isneeded for electrocoating compositions so that crater-free, smooth andeven finishes are formed on electrodeposition and curing. Chung et alU.S. Pat. No. 5,356,960 issued Oct. 18, 1994 shows an anticrateraddititve that forms a crater free, smooth and even finish. However,when this additive is used in an electrocoating composition that isbaked in an indirect gas oven after application to a metal substratesuch as an automobile or truck body, this anticrater additive migratesvery readily to the surface of the electrocoating composition duringbaking and any primer compositions applied over such a surfacecontaining polymeric melamine crosslinking agents adhere poorly to theelectrocoat composition and hence, adhesion failure of any topcoatapplied over the primer is readily evident. An anticrater additive isneeded that will not migrate to the surface of the depositedelectrocoating composition during baking and should not adversely affectother properties such as the throwing power of the electrocoating bath,the curing of the deposited coating or the film properties of theresulting finish.

SUMMARY OF THE INVENTION

An improved aqueous cathodic electrocoating composition having a binderof an epoxy-amine adduct and a blocked polyisocyanate crosslinkingagent; wherein the improvement is the use of an anticrater agent whichis a silane terminated reaction product of polyoxyalkylene diamine andan isocyanato alkyl alkoxy silane which is hydrolyzed and the anticrateragent has a number average molecular weight ratio of about 2,000-5,000determined by Gel Permeation Chromatography (GPC) using polystyrene asthe standard.

DETAILED DESCRIPTION OF THE INVENTION

The novel anticrater agent is readily incorporated into theelectrocoating composition by dispersing it with a nonionic surfactantin water and then adding it to an aqueous electrocoating compositionsince it is compatible with the other constituents of the composition.The anticrater agent remains stable in the composition and in theelectrocoating bath for extended periods of time under conventional bathoperating conditions since it is not reactive with the otherconstituents in the composition. The anticrater agent significantlyreduces and often eliminates craters in electrodeposited coatings andforms smooth and even finishes does not migrate to the surface on bakingof the finish. Also, the additive does not adversely affect otherproperties of the electrocoating bath or finishes of the electrocoatingcomposition. Further, the anticratering agent can be used as a rheologycontrol agent to improve edge protection of an electrodeposited finish.

The anticrater additive is used in an electrocoating composition in asufficient amount to significantly reduce or eliminate cratering in theelectrodeposited finish. Generally, the anticrater agent is used in theelectrocoating composition at a level of at least 0.5% by weight, basedon the total weight of binder solids in the electrocoating compositionand preferably, it is used at a level of about 0.5-10% by weight. Morepreferably, about 1-5% by weight of the anticrater agent is used. Thebinder of the electrocoating composition typically is a blend of anepoxy amine adduct and a blocked polyisocyanate crosslinking agent.

The anticrater agent is prepared by reacting a polyoxyalkylene diaminewith isocyanato alkyl alkoxy silane in a 1:2 molar ratio to form ananticratering agent having terminal silane groups. These constituentsare reacted at a temperature of about 70 to 130° C. for about 1 to 5hours until there is no residual isocyanate present and subsequentlyhydrolyzed. The anticrater agent has a number average molecular weightof about 2,000-5,000.

The polyoxyalkylene diamine used to form the anticrater agent has 2-4carbon atoms in the alkylene group and preferably is polyoxypropylenediamine having number average molecular weight of about 230-3,000preferably, 1,500-2,500 such as Jeffamine D-2000® having a numberaverage molecular weight of about 2000 available from Texaco ChemicalCompany. Another polyoxyalkylene diamine that can be used ispolyoxyethylene diamine having a similar molecular weight.

Typically useful isocyanato alkyl alkoxy silanes have the formulaO═C═N(CH₂)_(n) Si(OR)₃ where R is methyl, ethyl, or a mixture of methyland ethyl and n is 1-3. Typical silanes are isocyanto propyl trimethoxysilane, isocyanato ethyl trimethoxy silane, isocyanato methyl trimethoxysilane, isocyanto methyl triethoxy silane, isocyanato ethyl triethoxysilane, isocyanto propyl triethoxy silane. Isocyanoto propyl trimethoxysilane is preferred to form a high quality anticrater agent.

The anticrater agent can be added to the electrocoating composition atalmost any time. It can be added to the principal emulsion, or to thebath the anticrater agent is blended with a nonionic surfactant and anacid such as lactic acid and dispersed in water until hydrolysis ofsilane group to silanol groups is completed and then added to theelectrocoating composition as indicated above.

The anticrater agent after complete hydrolysis has the followingstructural formula: ##STR1## where n is 1-3 and m is 5-40 and R¹ is analkyl group having 2-4 carbon atoms.

Most principal emulsions used in an electrocoating composition comprisean aqueous emulsion of a binder of an epoxy amine adduct blended with acrosslinking agent which has been neutralized with an acid to form awater soluble product.

The anticrater agent is potentially usable with a variety of differentcathodic electrocoat resins, but the preferred resin is the typicalepoxy-amine adduct of the prior art. These resins are generallydisclosed in U.S. Pat. No. 4,419,467 which is incorporated by reference.

Typical acids used to neutralize the epoxy-amine adduct to form waterdispersible cationic groups are lactic acid, acetic acid, formic acid,sulfamic acid, alkane sulfonic acids such as methane sulfonic acid andthe like.

Preferred crosslinkers for the above resins are also well known in theprior art. These are aliphatic, cycloaliphatic and aromatic isocyanatessuch as hexamethylene diisocyanate, cyclohexamethylene diisocyanate,toluene diisocyanate, methylene diphenyl diisocyanate and the like.These isocyanates are pre-reacted with a blocking agent such as oximes,alcohols, or caprolactams which block the isocyanate functionality,i.e., the crosslinking functionality. Upon heating the blocking agentsseparate, thereby providing a reactive isocyanate group and crosslinkingoccurs. Isocyanate crosslinkers and blocking agents are well known inthe prior art and also are disclosed in the aforementioned U.S. Pat. No.4,419,467.

The cathodic binder of the epoxy amine adduct and the blocked isocyanateare the principal resinous ingredients in the electrocoating compositionand are usually present in amounts of about 30 to 50% by weight ofsolids of the composition. To form an electrocoating bath, the solidsare generally reduced with an aqueous medium.

Besides the binder resin described above, the electrocoating compositionusually contains pigment which is incorporated into the composition inthe form of a pigment paste. The pigment paste is prepared by grindingor dispersing a pigment into a grinding vehicle and optional ingredientssuch as wetting agents, surfactants, and defoamers. Any of the pigmentgrinding vehicles that are well known in the art can be used or theanticrater agent of this invention can be used. After grinding, theparticle size of the pigment should be as small as practical, generally,the particle size is about 6-8 using a Hegman grinding gauge.

Pigments which can be used in this invention include titanium dioxide,basic lead silicate, strontium chromate, carbon black, iron oxide, clayand the like. Pigments with high surface areas and oil absorbenciesshould be used judiciously because these can have an undesirable affecton coalescence and flow of the electrodeposited coating.

The pigment to binder weight ratio is also important and should bepreferably less than 0.5:1, more preferably less than 0.4:1, and usuallyabout 0.2 to 0.4:1. Higher pigment to binder weight ratios have beenfound to adversely affect coalescence and flow.

The coating compositions of the invention can contain optionalingredients such as wetting agents, surfactants, defoamers and the like.Examples of surfactants and wetting agents include alkyl imidazolinessuch as those available from Ciba-Geigy Industrial Chemicals as "AmineC", acetylenic alcohols available from Air Products and Chemicals as"Surfynol 104". These optional ingredients, when present, constitutefrom about 0.1 to 20 percent by weight of binder solids of thecomposition.

Optionally, plasticizers can be used to promote flow. Examples of usefulplasticizers are high boiling water immiscible materials such asethylene or propylene oxide adducts of nonyl phenols or bisphenol A.Plasticizers are usually used at levels of about 0.1 to 15 percent byweight resin solids.

The electrocoating composition of this invention is an aqueousdispersion. The term "dispersion" as used within the context of thisinvention is believed to be a two-phase translucent or opaque aqueousresinous binder system in which the binder is in the dispersed phase andwater the continuous phase. The average particle size diameter of thebinder phase is about 0.1 to 10 microns, preferably, less than 5microns. The concentrations of the binder in the aqueous medium ingeneral is not critical, but ordinarily the major portion of the aqueousdispersion is water. The aqueous dispersion usually contains from about3 to 50 percent preferably 5 to 40 percent by weight binder solids.Aqueous binder concentrates which are to be further diluted with waterwhen added to an electrocoating bath, generally have a range of bindersolids of 10 to 30 percent weight.

The following example illustrates the invention. All parts andpercentages are on a weight basis unless otherwise indicated.

EXAMPLE

Preparation of Anticrater Agent

The anticrater agent was prepared by charging 999.7 parts of JeffamineD2000® (polyoxypropylene diamine having a number molecular weight of2000) and 0.04 parts of dibutyl tin dilaurate into a suitable reactionvessel and heated to 37° C. under a dry nitrogen blanket. 205 parts ofisocyanato propyltrimethoxy silane was slowly charged into the reactionvessel while maintaining the reaction mixture below 82° C. for anadditional hour until essentially all of the isocyante was reduced asindicated by infrared scan of the mixture. The adduct was then dispersedby mixing in an aqueous medium of 5288.33 deionized water, 58.73 partsof lactic acid and 1174.62 parts of ethoxylated styrenated phenol(Synfac 8334®) and agitated for at least two hours for completehydrolysis of silane groups to silanol groups. The resulting adductsolution had a nonvolatile content of 30%.

Preparation of Chain Extended Polyepoxide Solution

The following ingredients were charged into a suitable reaction vessel:1478 parts of Epon 828® (Epoxy resin of diglycidyl ether of bisphenol Ahaving an epoxy equivalent weight of 188); 427 parts bisphenol A; 533parts ethoxylated bisphenol A having a hydroxy equivalent weight of 247(Synfac 8009®) and 121 parts xylene. The resulting reaction mixture washeated to 160° C. under nitrogen blanket and held at room temperaturefor 1 hour. 5.1 parts dimethyl benzyl amine were added and the mixturewas held at 147° C. until an epoxy equivalent weight of 1050 wasobtained. The reaction mixture was cooled to 98° C. and 168 parts ofdiketimine (reaction product of diethylenetriamine and methyl isobutyketone having a nonvolatile content of 72.27%) and 143 parts of methylethanol amine were added. The resulting mixture was held at 120° C. for1 hour and then 727 parts of methyl isobutyl ketone were added. Theresulting resin solution had a nonvolatile content of 75%.

Preparation of Crosslinking Resin Solution

An alcohol blocked polyisocyanate crosslinking resin solution wasprepared by charging 317.14 parts of PAPI 2027® (methylene diphenyldiisocyanate), 47.98 parts of methyl isobutyl ketone and 0.064 parts ofdibutyl tin dilaurate into a suitable reaction vessel and heated to 37°C. under a nitrogen blanket. A mixture of 323.10 parts of diethyleneglycol mono butyl ether and 13.04 parts of trimethylolpropane was slowlycharged into the reaction vessel while maintaining the reaction mixturebelow 93° C. for an additional hour until essentially all of theisocyanate was reacted as indicated by infrared scan of the reactionmixture. 2.30 parts of butanol and 167.37 parts of methyl isobutylketone were added. The resulting resin solution had a nonvolatilecontent of 75%.

Preparation of Quaternizing Agent

The quaternizing agent was prepared by adding 87 partsdimethylethanolamine to 320 parts ethylhexanol half-capped toluenediisocyanate in the reaction vessel at room temperature. An exothermicreaction occured and the reaction mixture was stirred for one hour at80° C. 118 parts aqueous lactic acid solution (75% nonvolatile content)was then added followed by the addition of 39 parts 2-butoxyethanol. Thereaction mixture was held for about one hour at 65° C. with constantstirring to form quaternizing agent.

Preparation of Pigment Grinding Vehicle

The pigment grinding vehicle was prepared by charging 710 parts Epon829® (diglycidyl ether of bisphenol A having an epoxide equivalentweight of 193-203) and 290 parts bisphenol A into a suitable vesselunder nitrogen blanket and heated to 150-160° C. to initiate anexothermic reaction. The exothermic reaction was continued for about onehour at 150-160° C. The reaction mixture was then cooled to 120° and 496part of 2-ethylhexanol half capped toluene diisocyanate was added. Thetemperature of the reaction mixture was held at 110-120° C. for onehour, followed by the addition of 1095 parts of 2-butoxyethanol, thereaction mixture was then cooled to 85-90° C. and then 71 parts ofdeionized water was added followed by the addition of 496 partsquarternizing agent (prepared above). The temperature of the reactionmixture was held at 85-90° C. until an acid value of about 1 wasobtained.

I. Preparation of Emulsion

    ______________________________________                           Parts by Weight    ______________________________________    Chain extended polyepoxide solution (prepared above)                             1255.31    Crosslinking resin solution (prepared above)                             805.85    Surfactant.sup.1         13.62    Lactic Acid              27.24    Deionized water          1897.98    Total                    4000.00    ______________________________________     .sup.1 Surfactant  120 parts Amine ® C from Ciba Geigy, 120 parts     acetylenic alcohol available as Surfynol ® 104 from AirProducts and     Chemicals, Inc., 120 parts 1butoxyethanol, 221 parts of deionized water     and 19 parts glacial acetic acid.

The chain extended polyepoxide solution, crosslinking resin solutionsurfactant and lactic acid were thoroughly mixed. The deionized waterwas then added under agitation. The emulsion nonvolatile content wasadjusted to 36% with the necessary amount of deionized water. Theemulsion was kept agitated until the methyl isobutyl ketone hadevaporated.

II. Preparation of Pigment Paste

    ______________________________________                        Parts by Weight    ______________________________________    Pigment grinding vehicle (prepared above)                          812    Deionized water       1660    Titanium dioxide pigment                          1068    Aluminum silicate pigment                          212    Lead silicate pigment 92    Carbon black pigment  32    Dibutyl tin oxide     124    Total                 4000    ______________________________________

The above ingredients were mixed until homogenous mixture was formed ina suitable mixing container. Then were dispersed by charging the mixtureinto a sand mill and then grinding until Hegman reading of seven orgreater was obtained.

III. Preparation of Electrocoating Baths I and II

    ______________________________________                       Parts by Weight                       Bath I                             Bath II    ______________________________________    Emulsion (prepared above)                         1636    1569    Deionized water      1926    1913    Pigment Paste (prepared above)                         398     398    Anticrater Agent (prepared above)                         40      120    Total                4000    4000    ______________________________________

A Cationic electrocoating Bath I and II were prepared by mixing theabove ingredients. Each bath was then ultrafiltered. Each bath waselectrocoated at 250-270 volts to obtain 0.9-1.0 mils (22.86-25.4microns). ASPP blow out crater test is used to test each bath. Craterresistance was rated according to the following rating scale of A-E:

A--0-10% defects

B--11-20% defects

C--21-40% defects

D--41-80% defects

E--Greater than 80% defects

The crater resistant rating for both Baths I and II was A.

An electrocoating bath was prepared identical to Bath I above except theanticrater agent was replaced with a conventional anti cratering agentwhich is the reaction product of Jeffamine® 2000 and Epon® 1001 epoxyresin and the crater resistance of this bath was tested as above byusing the ASPP blow out crater test. The crater resistance rating forthis panel was E which is substantially inferior to the panels coated inBaths I and II which contained the anticrater agent of this invention.

In order to measure edge corrosion resistance, knife blades (10×2 cm)were electrocoated in Baths I and II at 250 volts and baked at 182° C.(metal temperature) for 10 minutes. The knife blades then were exposedto salt spray for 7 days. The number of rust spots on each blade werecounted by viewing the blades under a microscope. The blades from Bath Ihad 100-120 rust spots while the blades from Bath II had 40-50 rustspots which shows that Bath II which contained three times the amount ofanticratering agent provided better edge corrosion protection.

I claim:
 1. An anticrater agent consists of the structural formula:##STR2## where n is 1-3 and m is 5-40 and R¹ is an alkyl group having2-4 carbon atoms.